fix(sidebar): add icon to expanded section, fix icon-rail callbacks, fix active border

- Render icon between chevron and label in expanded SidebarSection
- Remove !open guard from icon-rail click — always fire both callbacks
- Add border-left: 3px solid transparent to .treeSection so
  .treeSectionActive border-left-color takes effect
- Remove duplicate .treeSectionLabel CSS declaration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/layout/Sidebar/Sidebar.module.css

@@ -205,15 +205,7 @@
   padding: 0 6px 6px;
   margin-bottom: 2px;
   border-bottom: 1px solid rgba(255, 255, 255, 0.12);
-}
-
-.treeSectionLabel {
-  padding: 10px 12px 4px;
-  font-size: 10px;
-  font-weight: 600;
-  text-transform: uppercase;
-  letter-spacing: 1px;
-  color: var(--sidebar-muted);
+  border-left: 3px solid transparent;
 }
 
 /* Collapsible section toggle */